[clang] [llvm] Add length HLSL function to DirectX Backend (PR #101256)

[Farzon Lotfi via llvm-commits]

https://github.com/farzonl requested changes to this pull request.

You need test cases for the DXIL expansion pass and the  dxil-op-lower pass. It can be one file called `llvm/test/CodeGen/DirectX/length.ll`
It shold start off like:

```
; RUN: opt -S  -dxil-intrinsic-expansion  < %s | FileCheck %s --check-prefixes=CHECK,EXPCHECK
; RUN: opt -S  -dxil-op-lower  < %s | FileCheck %s --check-prefixes=CHECK,DOPCHECK
```
If you want an example see `llvm/test/CodeGen/DirectX/idot.ll`

https://github.com/llvm/llvm-project/pull/101256